2086
Ind. Eng. Chem. Res. 2005, 44, 2086-2090
Comparisons of Physical Properties of Intercalated and Exfoliated Clay/Epoxy Nanocomposites Ing-Nan Jan,† Tzong-Ming Lee,‡ Kuo-Chan Chiou,‡ and Jiang-Jen Lin*,† Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan, and Material Research Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan
Two types of intercalated and exfoliated silicate platelets were allowed to disperse in phenolcured epoxy matrixes. The different dispersing forms of these silicate platelets in the composites may alter their hardness, transparency, thermal stability, and coefficient of thermal expansion (CTE). In particular, the presence of silicate platelets in the epoxy substantially enhances the hardness of the pristine epoxy from 2 to 6 H by loading only 0.5 wt % of the exfoliated platelets but only to 4 H hardness by loading the intercalated form (pristine 58 Å d spacing). Analyses by X-ray diffraction, transmission electron microscopy (TEM), differential scanning calorimetry, thermal mechanics, and thermal gravimetry were performed to characterize these silicate-added epoxies. Physical properties including CTE and Tg were correlated with the silicate platelet dispersion in a matrix revealed by the TEM observations. Introduction The presence of inorganic silicates in organic polymers may reinforce the composite’s properties such as mechanical strength,1-3 gas barrier,3,4 and thermal stability.5,6 Among many inorganic fillers, layered silicate clays such as montmorillonite have been intensively studied in preparing nanocomposites. Generally, the naturally occurring clays are of hydrophilic character and require a modification by intercalating with amino acids, alkylammonium, or phosphonium salts to become organically compatible.7,8 The modified clays are actually organically encapsulated with a widened interlayer spacing of up to 30 Å. These intercalated organoclays are embedded with organic surfactants but still retain their layered structure. It relays the final process, either an in situ monomer polymerization or a direct polymer mixing at elevated temperature, to exfoliate the silicate platelets in a random form and in mixing with a polymer matrix. As a result, the presence of high aspect-ratio silicates significantly improves the physical properties of the polymers. The Toyota research group reported the first example of commercialized nanocomposites involving montmorillonite clay in Nylon 6.5,9 Since then, there have been numerous reports regarding the study of various polymer/layered silicate nanocomposites.7-15 However, the problems concerning the uniformity of silicate platelet dispersion in polymer matrixes still remain. The commonly utilized montmorillonite clay has a structure with multiple layers, approximately 8-10 platelets in a primary stack and 100 × 100 × 1 nm for each platelet.1-4 Because of their intensive ionic interaction between the neighboring platelets in the layered structure, the complete exfoliation of individual silicate platelets and their further homogeneous dispersion in a polymer matrix are difficult to achieve. Previously, we have reported the use of high-molecular-weight (2000-4000 g/mol) poly(oxyalkylene)-back* To whom correspondence should be addressed. Tel.: +886-4-2285-2591. Fax: +886-4-2287-1787. E-mail: jjlin@ dragon.nchu.edu.tw. † National Chung Hsing University. ‡ Industrial Technology Research Institute.
boned quaternary ammonium salts as intercalating agents for enlarging the clay interlayer space as wide as 58-92 Å.16-19 This interlayer spacing promotes the subsequent exfoliating process when mixing with the target polymers. The sufficient interlayer widening is the key condition for homogeneously dispersing silicate platelets. As exemplified in an amine-cured epoxy system, the enhancement of physical properties has been observed.18 In our recent study, it is shown that, by using a synthesized poly(oxyalkylene)amine Mannich oligomer (AMO) as the intercalating agent, sodium montmorillonite (Na+-MMT) can be directly exfoliated into random silicate platelets.20,21 The exfoliated silicate platelets can then be well-dispersed in organic monomers or polymers. The preparation of the intercalated and exfoliated clays allows us to compare their relative effectiveness on enhancing the nanocomposite’s physical properties by relating the results to the degree of platelet dispersion. In this paper, we demonstrate two different systems of the spatially enlarged and exfoliated silicates in mixing with epoxy polymers. The significantly improved hardness, coefficient of thermal expansion (CTE), and other properties are correlated with the silicate platelet dispersion revealed by the corresponding transmission electron microscopy (TEM) micrographs. Experimental Section Materials. Na+-MMT, a Na+ type of smectite clay with a cation exchange capacity of 1.15 mequiv/g, was supplied by Nanocor Co. and poly(oxyalkylene)amines, purchased from Aldrich Chemical Co. or Huntsman Chemical Co., include poly(oxypropylene) (POP)-backboned diamines. The Jeffamine POP2000 diamine is poly(propylene glycol)-bis(2-aminopropyl ether) with a molecular weight (Mw) of 2000 g/mol or 33 oxypropylene units. Liquid diglycidyl ether of bisphenol A (DGEBA, trade name BE-188) with an epoxide equivalent weight (EEW) of 188 was obtained from Chang Chun Chemical Co., Taiwan. A phenol-type curing agent (aminotriazine novolac, trade name LA-7751), with an N-H equivalent weight of 135, was supplied from Dainippon Ink &
10.1021/ie048934+ CCC: $30.25 © 2005 American Chemical Society Published on Web 02/15/2005
Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2087 Scheme 1. Process Flow Chart for the Preparation of Clay/Epoxy Nanocomposites
Figure 1. Chemical structure of DGEBA and the melaminenovalac curing agent.
Chemicals and Chang Chun Echo Chemical Co., Taiwan. The chemical structures of epoxy resin and curing agent are shown in Figure 1. Characterization and Instruments. X-ray diffraction (XRD) was recorded on a Shimadzu SD-D1 diffractometer with a Cu target (k ) 1.5405 Å). The basal spacing (n ) 1) was calculated according to Bragg’s equation (nλ ) 2d sin θ) through the observed peaks of n ) 2, 3, etc. Thermal gravimetric analysis (TGA) was performed on a Seiko SII model SSC/5200, by heating the samples from 50 to 850 °C at 10 °C/min in air. Transmission electronic microscopy (TEM) was performed on a Zeiss EM 902A at an acceleration voltage of 80 kV, and the samples with a thickness of approximately 80 nm were microtomed at room temperature using a diamond knife on a Reichert-Jung Ultracut UCT and transferred to carbon-film-coated Cu grids. Thermal mechanical analysis (TMA) was performed on a DuPont Instrument TMA2940 at heating or cooling rate of 10 °C/min under N2 and used to identify the CTE. Ultraviolet-visible (UV-vis) spectra were measured at a film thickness of 100 µm on a Perkin-Elmer Lambda 20 UV-vis spectrophotometer. The pencil hardness was measured according to the method of ASTM D 3363-74. The glass transition temperature (Tg) was measured by using differential scanning calorimetry (Perkin-Elmer DSC 7), under the conditions of using a second heating scan at 10 °C/min from 25 to 200 °C, and confirmed by TMA. All samples were prepared in films or bulk by casting on a flat aluminum-foil mould: films for XRD, UV-vis, and hardness test and bulk for TMA and TEM. Preparation of MMT/POP2000 and MMT/AMO Organoclays. The preparation of the POP2000-intercalated and AMO-exfoliated montmorillonite has been reported elsewhere.19-21 Examples of the experimental procedures are described below. Na+-MMT (10 g, 11.5 mequiv) was dispersed in 1 L of deionized water at 80 °C by vigorous mixing. POP2000 (23.0 g, 23.0 mequiv) and aqueous hydrochloric acid (37 wt %, 1.2 g, 11.5 mequiv) were dissolved in 30 mL of ethanol to form quaternary ammonium salts at room temperature and then poured into a Na+-MMT slurry in water. The mixture was continuously stirred at 70-75 °C for 5 h. A white precipitate was filtered, collected, and washed thoroughly with 400 mL of hot water/ethanol several times. The product was characterized by using XRD of 58 Å of basal spacing. Similarly, the MMT/AMO organoclay is used to intercalate montmorillonite with typical experimental procedures described below. Na+-MMT (10 g, 11.5 mequiv) was dispersed in 1 L of deionized water at 80 °C by vigorous mixing. The prepared AMO (57.5 g, 2 mol) and aqueous hydrochloric acid (37 wt %, 1.2 g, 11.5 mequiv) were dissolved in 50 mL of deionized water to form quaternary ammonium salts at room temperature and then poured into a Na+-MMT slurry in water. The
mixture was continuously stirred at 80-85 °C for an additional 5 h. The precipitate was filtered, collected, and washed thoroughly with 400 mL of water/ethanol several times. The product was dried in vacuo at 70 °C. XRD has demonstrated the randomness of the layered structure with the observation of no significant peak presence. Preparation of Clay/Epoxy Nanocomposites. A process flowchart for preparing the organoclay/epoxy/ curing agent nanocomposites is depicted in Scheme 1. Different weight fractions of organoclay were first dispersed in a curing agent. As an example, the MMT/ POP2000 hybrid, consisting of 63 wt % of organics and 37 wt % of silicates, was used in mixing with epoxy materials. The epoxy system is comprised of two components, DGEBA and curing agent (LA-7751), at a 1/0.4 equivalent weight ratio of epoxide/N-H to achieve optimum curing. The cured epoxy materials with the added 0.5, 1, 3, and 5 wt % of MMT/POP2000 or MMT/ AMO organoclays were prepared in films or bulk by casting on a flat aluminum-foil mould. The curing condition was programmed to be 180 °C in an oven for 3 h. The maximum 5 wt % is the limitation for the silicate loading because, above that, the viscosity of the epoxy component was too high for hand mixing homogeneously. The prepared epoxies were examined by using XRD analyses. Results and Discussion The spatially enlarged organoclays have been previously prepared by using POP-backboned quaternary ammonium salts of 400-2000 Mw as the intercalating agents for Na+-MMT. The wide d spacing, ranging from 19.4 to 58.0 Å, was correlated to the intercalant Mw’s. In using the 2000 Mw POP-amine quaternary salt, the intercalated MMT is composed of layered silicates with a basal spacing of up to 58 Å and incorporated organics of up to 63 wt %. The direct exfoliation of Na+-MMT is also possible when using POP-amine-segmented oligomers as the intercalating agents. The layered platelets were delaminated and dispersed when mixed in the exfoliating agents. Two forms of the intercalated and exfoliated silicates are employed to compare the effectiveness of silicate platelets distributed in a polymer matrix.
2088
Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005
Figure 2. XRD patterns of (a) MMT/POP2000, (b) 1 wt % of MMT/ POP2000 in epoxy, (c) 3 wt % of MMT/POP2000 in epoxy, (d) MMT/ AMO, (e) 1 wt % of MMT/AMO in epoxy, (f) 3 wt % of MMT/AMO in epoxy, and (g) pure epoxy.
XRD Analyses. XRD analyses were used to characterize the crystallographic structure of the modified clays and their dispersed homogeneity in an epoxy matrix. Generally, the wide expansion of the d spacing in the layered silicates indicates a large amount of organic encapsulation in addition to a compatibility with the target polymers. A lack of diffraction peaks in the XRD pattern occurs because of the randomization or exfoliation of the clay. As shown in Figure 2a, the XRD pattern of the blank MMT/POP2000 organoclay demonstrates a d spacing of 58 Å, which is calculated on the basis of Bragg’s equation (nλ ) 2d sin θ and the observed peaks of n ) 2, 3, etc.). Upon the addition of 1 and 3 wt % of the organoclay, the cured epoxy materials exhibit none of the diffraction peaks in the 2-10° angle (2θ) range in Figure 2b,c, which implies a disordered form of the original 58 Å d spacing silicates at least to the degree beyond the XRD detecting limit. By comparison, the neat MMT/AMO organoclay is an exfoliated silicate material with no XRD diffraction pattern (Figure 2d). With the addition of 1 and 3 wt % to the epoxy (Figure 2e,f), the cured materials show no XRD peak in the range of 2-10° angle (2θ). It appears that the silicates remain as a fine dispersion in the epoxy. Furthermore, the broad peak at an 18° angle (2θ) disappears completely for the 3 wt % MMT/AMO addition, as shown in Figure 2f. It is assumed that the disappearance of the pristine epoxy peak is an indication of the fine dispersion and intensive interaction of MMT platelets with the epoxy matrix. This phenomenon is observed only with the exfoliated silicate clay at higher percentages of the MMT/AMO addition. The epoxy matrix could have shifted from its original crystalline phase (Figure 2g) to the silicate platelet interacted phase, although further evidence is lacking. TEM Micrographs of Silicates in the Epoxy System. Although XRD analyses indicated a lack of ordered silicate alignments in the epoxy matrix, the detail picture of silicate platelet dispersion can be further revealed by using the analysis of TEM. The neat MMT/POP2000 (58 Å d spacing by XRD analysis) has a primary structure of 8-10 parallel silicate platelets
with a maximal length of approximately 100 nm for individual platelet dimensions.16 With 3 wt % loading in the cured epoxy, the intercalated MMT/POP2000 organoclay was only partially delaminated, as shown in Figure 3a,b at different magnifications. In the magnified micrograph, some of the layered silicates still remained in 2 or 3 layers or even the presence of a primary intercalated stack. The arrows show a 3-layer silicate stack of 5.8 nm spacing and separated layers of approximately 10 nm spacing. By comparison, the 3 wt % loading of the exfoliated MMT/AMO organoclay in the epoxy demonstrated a better dispersion in Figure 3c,d. The uniform dispersion of the exfoliated silicate layers is finely distributed in the epoxy matrix, although some structures of an average of 1 or 2 silicate platelet lamellae with a spacing of 10-25 nm are shown in Figure 3d. In general, the layered silicates are shown to be exfoliated into individual platelets more homogeneously for MMT/AMO than for MMT/POP2000 in epoxy matrixes, although some aligned platelets are still in existence in both epoxy samples. Thermal Property Analysis by TGA. The TGA curves of the pure epoxy and the silicate-added epoxies at 10 °C/min under air are shown in Figure 4. The thermo-oxidative degradation of these epoxies is comprised of at least two steps in the oxidation at high temperature because of the presence of different types of organic components in the epoxy structure. The temperature of degradation at which the weight loss is 10% for pure epoxy and nanocomposites is between 330 and 359 °C. The thermal stability of the nanocomposites is not superior to that of the pure polymer at low temperature. However, when the temperature is increased to 450-550 °C, which results in a weight loss of 40-50%, the modified epoxy nanocomposites demonstrate a good thermal stability. The result shows an improvement in thermal stability of over 100 °C in the high-temperature period, in the 5 wt % of clay-added epoxy. Furthermore, flame resistance can be evaluated when the temperature reaches 600-650 °C. The exfoliated MMT/AMO addition shows an improved thermodelay in comparison with the MMT/POP2000 addition. This characteristic behavior may be due to the different degrees of dispersion between MMT/AMO and MMT/POP2000 in epoxies. Furthermore, the addition of inorganic materials has resulted in a char yield of 4.6 wt % at 900 °C with 5 wt % clay loading. Physical Properties. The film samples of MMT/ POP2000- and MMT/AMO-added epoxies were prepared. As summarized in Table 1, the hardness of these epoxy nanocomposites was shown to be significantly improved by adding silicates of different forms. The pure epoxy was cured according to a formulation involving DGEBA and a melamine-novalac phenol curing agent. With the organoclay additions, the hardness of nanocomposites is enhanced tremendously from the pristine 2 H to the 4-8 H depending on the amount of silicates added. There are differences between the kinds of organoclays, the intercalated MMT/POP2000, and the exfoliated MMT/AMO. By comparison, the reinforcement benefit is substantially greater by adding MMT/ AMO than by adding MMT/POP2000. The improvements of 6 H versus 4 H for a 0.5 wt % clay loading and 7 H versus 5 H for a 1-3 wt % loading are compared. The difference in hardness is attributed to the strong interaction between platelets and the epoxy matrix and corresponded to the degree of platelet dispersions as
Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2089
Figure 3. Comparative TEM micrographs: (a and b) MMT/POP2000 (3 wt %) in epoxies; (c and d) MMT/AMO (3 wt %) in epoxies at different magnifications. Table 1. Properties of Films Prepared from the Addition of Different Organoclays to Epoxies organoclay none MMT/POP2000 (XRD ) 58 Å) MMT/AMO (XRD, featureless)
MMT content (wt %)a
hardness (H)
transparency (%)b
CTE (µm/m °C)c
Tg (°C)d
0 0.5 1 3 5 0.5 1 3 5
2 4 5 5 6 6 7 7 8
60.0 59.3 58.2 56.8 55.4 57.5 56.1 50.3 43.6
66.9 52.0 33.5 32.3
113.3 137.9 136.2 134.4
47.0 37.5 28.5
140.4 138.8 137.4
a Calculated on the basis of silicates. b Determined by using a UV-vis absorption method at 550 nm/T %. c CTE: coefficient of thermal expansion, determined by TMA. d Based on DSC measurements.
revealed by TEM pictures. The material transparency was characterized by using optical UV-vis spectroscopy at an absorbance of 550 nm. Maintenance of a 90% T of the original transparency for the nanocomposites with 0.5-1.0 wt % silicate additions is considered to be translucent for applications. Localized aggregation caused by the strong platelet charge interaction at high concentration (>3 wt %) of the exfoliated platelets (MMT/ AMO) may result in the reduction of light transparency. The CTE in units of µm/m °C, determined by TMA of samples under stress and deformation as a function of temperature, was found to be generally decreased with silicate additions in the range of 0.5-3 wt % loading. The intense interaction of organics and silicate platelets
renders the polymer matrix resistant to the temperature change. On the basis of DSC measurements, the Tg’s are found to increase significantly from the pure epoxy of 113.3 °C to 137.9 and 140.4 °C for the MMT/POP2000 and MMT/AMO additions, respectively. The Tg is affected by the initial 0.5 wt % loading but levels off for the higher loadings. The leveling effect beyond the 0.5 wt % additions is explained by the introduction of excess organic components from the organoclays to the epoxies. Conclusions The degree of silicate platelet dispersion in epoxy matrixes is correlated to the nanocomposite properties.
2090
Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005
Figure 4. TGA curves of the silicate-added epoxies in air.
Both intercalated MMT/POP2000 (XRD d spacing, 58 Å) and exfoliated MMT/AMO (XRD featureless) are effective for the enhancing hardness, Tg, and CTE. Particularly, the hardness is significantly enhanced from 2 to 6 H by loading only 0.5 wt % of the exfoliated silicates but is less affected by the intercalated clay. Similar trends of property changes are observed for Tg and CTE. These results correlate well with the TEM micrographs, in which the different degrees of individual platelet dispersion in epoxies are revealed. Acknowledgment We acknowledge the financial support from the National Science Council (NSC) of Taiwan. Literature Cited (1) Wang, Z.; Pinnavaia, T. J. Nanolayer Reinforcement of Elastomeric Polyurethane. Chem. Mater. 1998, 10, 3769. (2) Xu, R.; Manias, E.; Snyder, A. J.; Runt, J. New Biomedical Poly(urethane urea)-Layered Silicate Nanocomposites. Macromolecules 2001, 34, 337. (3) Wang, Z.; Pinnavaia, T. J. Hybrid Organic-Inorganic Nanocomposites: Exfoliation of Magadiite Nanolayers in an Elastomeric Epoxy Polymer. Chem. Mater. 1998, 10, 1820. (4) Yano, K.; Usuki, A.; Okada, A. Synthesis and Properties of Polyimide-Clay Hybrid Films. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2289.
(5) Gilman, J. W. Flammability and Thermal Stability Studies of Polymer Layered-Silicate Clay Nanocomposites. Appl. Clay Sci. 1999, 15, 31. (6) Becker, O.; Varley, R.; Simon, G. Morphology, Thermal Relaxations and Mechanical Properties of Layered Silicate Nanocomposites Based upon High-Functionality Epoxy Resins. Polymer 2002, 43, 4365. (7) Kong, D.; Park, C. E. Real Time Exfoliation Behavior of Clay Layers in Epoxy-Clay Nanocomposites. Chem. Mater. 2003, 15, 419. (8) Xu, W. B.; Bao, S. P.; He, P. S. Intercalation and Exfoliation Behavior of Epoxy Resin/Curing Agent/Montmorillonite Nanocomposite. J. Appl. Polym. Sci. 2002, 84, 842. (9) Usuki, A.; Hasegawa, N.; Kadoura, H.; Okamoto, T. ThreeDimensional Observation of Structure and Morphology in Nylon6/Clay Nanocomposite. Nano Lett. 2001, 1, 271. (10) Kornmann, X.; Lindberg, H.; Berglund, L. A. Synthesis of Epoxy-Clay Nanocomposites: Influence of the Nature of the Clay on Structure. Polymer 2001, 42, 1303. (11) Zanetti, M.; Lomakin, S.; Camino, G. Polymer Layered Silicate Nanocomposites. Macromol. Mater. Eng. 2000, 279, 1. (12) Alexandre, M.; Dubois, P. Polymer-Layered Silicate Nanocomposites: Preparation, Properties and Uses of a New Class of Materials. Mater. Sci. Eng. 2000, 28, 1. (13) Brown, J. M.; Curliss, D.; Vaia, R. A. Thermoset-Layered Silicate Nanocomposites. Quaternary Ammonium Montmorillonite with Primary Diamine Cured Epoxies. Chem. Mater. 2000, 12, 3376. (14) Ishida, H.; Campbell, S.; Blackwell, J. General Approach to Nanocomposite Preparation. Chem. Mater. 2000, 12, 1260. (15) Ray, S. S.; Okamoto, M. Polymer/Layered Silicate Nanocomposites: A Review from Preparation to Processing. Prog. Polym. Sci. 2003, 28, 1539. (16) Lin, J. J.; Cheng, I. J.; Wang, R.; Lee, R. J. Tailoring Basal Spacings of Montmorillonite by Poly(oxyalkylene)diamine Intercalation. Macromolecules 2001, 34, 8832. (17) Chou, C. C.; Shieu, F. S.; Lin, J. J. Preparation, Organophilicity, and Self-Assembly of Poly(oxypropylene)amine-Clay Hybrids. Macromolecules 2003, 36, 2187. (18) Lin, J. J.; Cheng, I. J.; Chu, C. C. High Compatibility of the Poly(oxypropylene)amine-Intercalated Montmorillonite for Epoxy. Polym. J. 2003, 35 (5), 411. (19) Lin, J. J.; Lin, S. F. Phase Inversion of Self-Aggregating Mannich Amines with Poly(oxyethylene) Segments. J. Colloid Interface Sci. 2003, 258, 155. (20) Lin, J. J.; Chu, C. C.; Tsai, C. M. Direct Exfoliation of Montmorillonite by Amine Terminated Mannich Oligomers via a Zigzag Mechanism. Chem. Mater. 2004, submitted for publication. (21) Lin, J. J.; Chu, C. C.; Chou, C. C.; Shieu, F. S. SelfAssembled Nanofiber Arrays from Random Silicate Platelets. Adv. Mater. 2005, 17 (3), 301.
Received for review November 3, 2004 Revised manuscript received December 30, 2004 Accepted January 11, 2005 IE048934+
